Extensible boom cranes are well known and widely utilized for lifting loads in various situations. Transportable extensible boom cranes are particularly useful in that they may be easily located and relocated to where tasks are to be performed. The present invention relates to a structure for enhancing the safety of operation of extensible boom cranes, transportable or otherwise.
By way of background, FIG. 4 illustrates schematically a transportable extensible boom crane in the form of a truck mounted crane. Such an apparatus comprises generally a truck 1 having an extensible boom crane 2 mounted thereon, as is well known. The crane includes, generally, a base portion 3 mounted to the truck frame and an extensible boom. The boom includes a base section 4 pivotally mounted to base 3 and one or more extensible and retractable sections 5.
A boom tip 6 is mounted at the end-most portion of the furthest extensible section 5. As is well known, boom tip 6 carries one or more rotatable sheaves. A cable associated with the crane passes over the sheaves and supports a hook or other grasping element for raising and lowering loads in a well known manner.
Such cranes, mounted on trucks or other transportable vehicles, often include a system for monitoring the load lifted by the crane and/or the forces imposed by the load to assure that the capacity of the crane is not exceeded at any time. Such systems will be discussed in greater detail hereinafter. Generally, load monitoring systems include sensors for monitoring parameters such as the position of the crane boom, the length of the boom at any point in time, and other parameters indicative of the physical configuration of the crane. Load monitoring systems determine when the capacity of the crane might be exceeded for any configuration of the crane. Thus, such systems enhance the safety of operation.
In some situations, the configuration of the crane might be modified in a manner which could result in readings in a load monitoring system which do not fully represent the configuration of the crane. One such situation is when a jib extension is added to an extensible boom crane. Jib extensions are added, for example, to extend the reach of the crane when necessary or desirable.
FIG. 5 illustrates the manner in which a jib extension might be added to an extensible crane boom. Reference numeral 5 indicates the end-most extensible boom section of a crane generally as discussed with respect to FIG. 4. Normally, one or more sheaves are mounted on boom tip 6.
As illustrated in FIG. 5, a jib extension 20 is secured to boom section 5 in such a manner as to extend the reach of the crane boom. The extension 20 can be mounted so that is it extends generally along the same axial direction as boom section 5, as illustrated. Alternatively, extension 20 can be mounted so that it extends at an angle to the axis of section 5. The cable of the crane will extend past boom tip 6, along the length of the jib extension 20, and over a rotatable extension sheave 38, as will be discussed in greater detail hereinafter.
In a typical arrangement, as illustrated, jib extension 20 is secured to boom section 5 by, for example, a plurality of pins 7 passing through apertures in the jib extension and the boom tip. Such securing arrangements are well known and not part of the present invention.
A known way for monitoring loads on a crane and preventing overload is to install a Load Moment Indicator (LMI) system. The LMI is an electronic system equipped with sensors to measure such parameters as the length of the boom (via a cable reel mounted on the boom or comparable means), the angle of orientation of the boom (via an angle transducer), the hydraulic pressure in the boom lift cylinder, and other relevant parameters. For example, sensors may be provided to indicate the position of stabilizing outriggers in the event that the transportable crane is equipped with such outriggers. The LMI includes a computer for comparing input data from such sensors to capacity information for the crane stored electronically. Such comparison determines if the crane is overloaded when lifting a load in any specific configuration of the boom, outriggers, etc. If an overload condition occurs, a warning signal and/or a control signal can be provided to warn an operator or to curtail crane functions to prevent a dangerous condition.
LMI's are very expensive and are not economically reasonable for smaller cranes. Additionally, data relating to the capacity of a crane relates to the normal variables in the configuration of the crane measurable by the LMI sensors.
An alternative to a complete LMI system is a Hydraulic Capacity Alert system (HCA). The HCA uses a pressure switch in the boom lift cylinder to measure the hydraulic pressure in the lift cylinder required to raise the boom and support or lift the load on the hook. At a preselected pressure, presumed to represent a pressure at which the crane is loaded to an undesired level by the load on the hook, a solenoid valve or other similar device is triggered to terminate the flow of oil to crane functions and/or provide a warning signal to an operator. Terminating or otherwise controlling crane operation in a suitable manner when critical pressure is reached will prevent crane overload from occurring. Such a system is more economical than a full LMI, which makes it more feasible for smaller cranes.
However, an HCA as described requires that, for all configurations of the crane (i.e., all boom lengths, all orientations of the boom), the same hydraulic pressure in the lift cylinder must be regarded as the limiting load factor. This may prevent the crane from achieving optimal operating conditions in some configurations.
The HCA may not protect a jib extension itself from overload. When a jib extension is mounted to the end of an extensible crane boom and a load is lifted using the jib extension, it is conceivable that the load capacity of the jib extension itself could be exceeded while the load capacity of the crane, as a whole, is well within safe limits. Therefore, it is important to provide means for assuring that the load capacity of the jib extension itself is not exceeded.
Accordingly, it is an object of the invention to provide an auxiliary load limiting device for a lifting crane, especially for a jib extension for a boom of a lifting apparatus.
An object of the invention is to provide such a load limiting device which may be readily utilized with any such lifting apparatus. A further object is to provide such a load limiting device which is economical for cranes of any relative size and cost.